1. Field of the Invention
The present invention relates to new polyisocyanates containing reversibly blocked isocyanate groups and also 1,3,5-triazine structural units incorporated by urea groups, to a process for the preparation of these polyisocyanates and to their use in combination with organic hydroxyl compounds as stoving lacquers.
2. Description of the Prior Art
It is known to use 1,3,5-triazine derivatives, in particular melamine, as starting materials for the preparation of binders for coating compositions. Thus, N-methylolmelamine resins and alkoxylated derivatives thereof are known as cross-linking agents for coating compositions; see, for example, T. Yamamoto, T. Nakamichi and O. Ohe in J. of Coat. Technology 60, 51 (1988) or H. Durr and M. Schon in Kunstharznachrichten 45, 17 (1986).
It is also known to use melamine as a component of polyurethane resins or polyurethane coating compositions. Thus, isocyanate group-containing addition products of melamine and aromatic diisocyanates are described as incorporable fillers in DE-OS 2 737 402.
EP-A-56 153, on the other hand, describes epoxide group-containing compounds prepared from melamine-containing triisocyanates and glycidyl ethers as fillers with reduced melting points and improved compatibility which may be incorporated in polyurethane resins.
According to DE-OS 2 938 309, isocyanate prepolymers are reacted with free melamine used in the form of a polyester/melamine paste for the preparation of duroplastic coatings which have exceptionally good adherence to glass bottles.
Duroplastic resins prepared e.g. from diphenylmethane diisocyanate and melamine are described in DE-OS 3 609 687. Polycarbodiimide/melamine mixtures in the form of foams or powders which harden at 230.degree. C. to form heat resistant and flame resistant resins are prepared by a process including carbodiimidization of the isocyanate.
According to DE-OS 2 844 132, for example, reaction products of diisocyanates and melamine at an NCO/NH.sub.2 equivalent ratio of 1:1 are used as components for flame retardant coating compositions and according to Japanese Application JP 59 041-320 (Derwent 84-096 976) they are used as heat resistant resins.
Blocked polyisocyanates are also known. They are widely used in polyurethane chemistry for the production of polyurethane coating compositions. A comprehensive account of the chemistry of blocked isocyanates is given, for example, by Z. W. Wicks in Progress of Organic Coatings 3, 73-99 (1975) and 9, 3-28 (1981).
Polyisocyanates containing 1,3,5-triazine groups are high melting compounds which are difficult to dissolve in conventional organic solvents; see DE-OS 2 737 402, page 7, lines 23-25 and page 8, lines 15-19; EP-A-56 153, page 1, lines 12-15 and page 7, lines 24-27; and DE-OS 3 609 687, page 15, lines 18-20.
Although the incorporation of 1,3,5-triazine structural elements in many cases imparts advantageous properties to polyurethane resins and although there have been many attempts, as may be seen from the previously mentioned prior art, to make use of the advantageous properties of 1,3,5-triazine structural units in polyurethanes, polyisocyanates containing 1,3,5-triazine groups have previously hardly been used due to their high melting points and their poor solubility in organic solvents. Products which form clear solutions in conventional, physiologically harmless solvents are particularly in demand as coatings raw materials.
It is an object of the present invention to provide polyisocyanates which contain 1,3,5-triazine structural units and do not suffer from the disadvantages mentioned above. It is an additional object of the present invention to provide polyisocyanates which contain 1,3,5-triazine structural units and which have a low melting range and are soluble in conventional lacquer solvents. It is a further object of the present invention to provide polyisocyanates which contain 1,3,5-triazine structural units and which may be prepared by a simple process and react in the form of a solution or soluble, solid resin with isocyanate reactive compounds, in particular with polyhydroxyl compounds, to form high quality polyurethane resins, in particular coatings.
This object may be achieved in accordance with the present invention with the polyisocyanates described below. The term "polyisocyanates" is used in the context of this invention to denote both polyisocyanates, i.e., specific, individual compounds containing blocked isocyanate groups, and mixtures of polyisocyanates containing blocked isocyanate groups, which may be obtained by carrying out the process according to the invention.
The invention is based on the surprising observation that blocked polyisocyanates containing 1,3,5-triazine structural elements have a lower melting point and are more soluble in conventional lacquer solvents than the corresponding unblocked polyisocyanates. This fundamental difference between the blocked isocyanates and the unblocked polyisocyanates was not expected since it is not generally observed in the known polyisocyanates used in polyurethane chemistry.